In the drilling industry, the term well completion is often used to denote the operations that prepare a well bore for producing oil or gas from the reservoir. It may similarly refer to a completed wellhead assembly. The goal of these operations is to install a wellhead and other connections to optimize the flow of the reservoir fluids into the well bore, up through the producing string, and into the surface collection system.
To begin a drilling operation a conductor pipe may be driven into the ground to prevent the loose surface soil from caving into the hole as the upper portion of the borehole is being drilled. Various components are then attached to the conductor pipe. A single conductor for 2 or more well completions is often used as it provides benefits such as smaller platform sizes and reduced installation time. Each well completion requires one wellhead to be installed before oil/gas production can commence. Thus 2 or more separate wellheads may be provided in a single conductor.
FIG. 1 shows a cross-sectional view of one example of a prior art conductor pipe assembly, designated generally as reference numeral 10. FIG. 1A illustrates a close-up cross-sectional view of the circled portion “A” of FIG. 1. FIG. 2 illustrates a cross-sectional view of the assembly 10 of FIG. 1 with two wellheads attached. FIG. 3 illustrates a top perspective view of the conductor housing of FIGS. 1 and 2.
With continuing reference to FIGS. 1-3, the assembly 10 includes a conductor 12 and a conductor housing 20 mounted to a top 14 of the conductor 12. The conductor housing 20 includes two cylindrical holes 24a, 24b separated by a central section 26 that extends longitudinally into the central bore of the conductor 12. The two cylindrical holes 24a, 24b of the assembly 10 facilitate the drilling of 2 separate wells 6, 8 (represented graphically as the centerlines of the holes 24a, 24b) within the conductor 12. Typically, the following steps are required to complete the connection between the conductor housing 20 and each wellhead 30a, 30b (FIG. 2). Usually, only one well (6 or 8) is worked on and the other well 6, 8 is covered with a debris cap 16.
For example as shown in FIG. 1, during the installation phase, a riser 40 that is used to protect the well fluids from the environment is required to be installed before any drilling operations. In this example, the riser 40 is installed above well 6. One or more seals 42 are located at the bottom of the riser 40 between an outside surface 44 of the riser 40 and an inside surface 22 of the conductor housing 20. In this prior art assembly, the seals 42 directly contact the conductor housing 20.
A separate debris cap 16 is installed on the conductor housing 20 to protect well 8. As best shown in FIG. 1A, the debris cap 16 includes one or more seals 17 between the debris cap 16 and the inside surface 22 of the central portion 26 of hole 24a in the conductor housing 20. In this prior art wellhead, the riser 40 seals directly onto the conductor housing 20. The “Sealing Thickness”, shown as Ts, must be sufficient to hold pressure regardless of whether the riser 40 is installed in either of the cylindrical holes 24a, 24b above the well bores 6, 8. The thickness of the riser 40 is shown as Tr. The total available thickness for well drilling operations is shown as Ttotal. The portion marked Tw is “Wasted Thickness” which is there to provide for the riser 40 to be installed when drilling operations are switched to the other bore. When no drilling operations go through that bore, the area is dead space and is considered wasted. This wasted thickness Tw is undesirable.
After the well 6 is drilled through, a casing hanger 50 (FIG. 2) is installed, and a casing 55 is inserted into the well 6. The riser 40 is then dismantled. Subsequently, the wellhead 30a is installed onto the conductor housing 20. One or more seals 32 may be installed on an inside surface 34 of the wellhead 30a to provide a leak-proof connection to an outside surface 52 of the casing hanger 50.
In such a conductor splitter application, the wells 6, 8 have to be located close to each other, constrained by the internal diameter of the conductor 12, and the thickness Ttotal of the central section 26 of the conductor housing 20. Furthermore, the center to center distance 18 between well 6 and well 8 is constrained to allow two separate vertical bores to pass through the conductor housing 20 through cylindrical holes 24b, 24a, respectively. As the riser 40 and subsequently the wellhead 30a, 30b must be fitted within the boundary of each bore for well isolation, the wall thickness of the riser 40 is also constrained. The internal diameter of the riser 40 is also constrained by the minimum allowed diameter based on industry standards. Similarly, as the bottom of the riser 40 seals directly on the conductor housing 20, the central section 26 must be sufficiently thick to withstand the well pressure and allow sealing on either side of the bore. These constraints limit the amount of pressure under which the wells 6, 8 may operate. For example, in typical well completions as shown in FIGS. 1 and 2, each well may be constrained to operate at a pressure of 3000 psi (20.6 MPascal) or less.
One solution to increase the available pressure in the wellhead is to use a smaller drill bit that would allow for a thicker riser wall. However, using a smaller drill bit also results in a smaller casing size for the well. While the operating pressure of the resulting well may be increased, the overall volume is less than what would be produced using the larger drill bit at the higher pressure. This is often unacceptable to the operator of the well. An alternate solution is to provide a larger conductor, thus increasing the center to center distance between the wells, so that the original drill bit may be used, and appropriate high-pressure wellheads installed. This option may greatly increase the cost of the required wellhead equipment.
Yet another solution is to use an underreamer which is able to pass through the riser and subsequently expand the cutter arms to enlarge the borehole. However, this solution increases both the time required and the costs associated with the drilling operation.